The present invention pertains to a Salmonella microorganism having an attenuating mutation which disrupts the expression of a gene located within the Spi2 pathogenicity island, and an auxotrophic mutation. The microorganism therefore has a double mutation which helps prevent reactivity of the microorganism while maintaining the effectiveness of the microorganism to elicit an immune response. The present invention also pertains to vaccine compositions and methods for treating and preventing a Salmonella infection in a patient.